After leaving school he worked as a journalist for a variety of papers and journals including The Quiver and the Daily News and the Christian Commonwealth. Fenner Brockway joined the Independent Labour Party (ILP) in 1907 and was a regular visitor to the Fabian Society. He was appointed editor of the Labour Leader (the newspaper of the ILP, later called the New Leader) and was, by 1913 a committed pacifist. He opposed British involvement in the First World War and, through his position as editor of the Labour Leader, was outspoken in his views about the conflict. On 12 November 1914 he published an appeal for men of military age to join him in forming the No-Conscription Fellowship to campaign against the possibility of the government attempting to introduce conscription in Britain. The offices of the Labour Leader were raided in August 1915 and Brockway was charged with publishing seditious material. He pleaded not guilty and was acquitted in court. In 1916 Fenner Brockway was again arrested, this time for distributing anti-conscription leaflets. He was fined, and after refusing to pay the fine, was sent to Pentonville Prison for two months.[3]

Shortly after his release Fenner Brockway was arrested for a third time for his refusal to be conscripted, after being denied recognition as a conscientious objector. He was handed over to the Army and court-martialled for disobeying orders. As if a traitor, he was held for a night in the Tower of London, in a dungeon under Chester Castle and in Walton Prison, Liverpool, where he edited an unofficial newspaper, the Walton Leader for conscientious objectors in the prison. This led to his being disciplined, which in turn led to a 10-day prison strike by conscientious objectors before he was transferred to Lincoln Jail, where he spent some time in solitary confinement until finally released in 1919. In October 1950 he revisited the jail with Eamon de Valera, the Irish statesman.[4] Following his release he became an active member of the India League, which advocated Indian independence. He became secretary of the ILP in 1923 and later its chairman. In 1926, he became the first chairperson of War Resisters' International, serving in this post until 1934.[5]

In 1929, he was elected Member of Parliament for Leyton East as a Labour Party candidate. He polled 11,111 votes and, immediately after the election, the Liberal candidate announced that Fenner Brockway had converted him to socialism. His convictions brought him into difficulties with the Labour Party. He was also outspoken in Parliament, and was once "named" (suspended) by the Speaker while demanding a debate on India at Prime Minister's Question Time.[6]

In 1931 Brockway lost his seat and the following year he disaffiliated from the Labour Party along with the rest of the ILP. He stood unsuccessfully for the ILP in a 1934 West Ham by-election and in Norwich in the 1935 election. He also wrote the well-known book on the arms trade, The Bloody Traffic, published by Gollancz Ltd in 1934. According to David Howell, after 1932 Brockway "sought to articulate a socialism distinct from the pragmatism of Labour and the Stalinism of the Communist Party".[7]

With the rise of fascism in Spain, Fenner Brockway began to believe that it might be necessary to fight to preserve the peace in the long run. Despite his previous pacifist commitment, he resigned from War Resisters' International, explaining that he was "faced by this fact: If I were in Spain at this moment I should be fighting with the workers against the Fascists forces. I believe it to be the correct course to demand that the workers shall be provided with the arms which are being sent so freely by the Fascist powers to their enemies. I appreciate the attitude of the pacifists in Spain who, whilst wishing the workers success, feel that they must express their support in constructive social service alone. My difficulty about that attitude is that if anyone wishes the workers to be triumphant he cannot, in my view, refrain from doing whatever is necessary to enable that triumph to take place".[8]

He assisted in the recruitment of British volunteers to fight the fascist forces of Francisco Franco in Spain through the ILP Contingent. He sailed to Calais in Feb 1937 and was believed to have been destined for Spain.[9] Among those who went to Spain was Eric Blair (better known as George Orwell) and it is known that Fenner Brockway wrote a letter of recommendation for Blair to present to the ILP representatives in Barcelona. Following the Spanish Civil War, he advocated public understanding of the conflict. He wrote a number of articles about the conflict and was influential in getting Orwell's Homage to Catalonia published.[10]

Notwithstanding his support for British participation in the Second World War, he served as Chair of the Central Board for Conscientious Objectors throughout the war, and continued to serve as Chair until his death.[11]

Following the war Fenner Brockway rejoined the Labour Party. In 1950 he won the House of Commons seat of Eton and Slough.

In 1951 he was one of the four founders of the charity War on Want, which fights global poverty. He helped establish the Congress of Peoples Against Imperialism (est. 1945), an organisation he continued to work with throughout the 1950s.[12] His activities there included protesting against the response of the colonial government to the Mau Mau Uprising events in Kenya during this period.[13] In this area, he was a part of the larger Movement for Colonial Freedom. From the late 1950s he regularly proposed legislation in Parliament to end racial discrimination, only to be defeated each time. He strongly opposed the use or possession of nuclear weapons by any nation and was a founding member of the Campaign for Nuclear Disarmament.[citation needed]

He narrowly lost his seat in the House of Commons at the 1964 election, despite the national swing to Labour at that election, as he was portrayed by his opponents as being the principal cause of immigrants from the West Indies settling in Slough.[15] He subsequently accepted a life peerage as The Baron Brockway, of Eton and Slough in the Royal County of Berkshire, and took a seat in the House of Lords.

While he was in prison, Brockway met the prominent peace activist Stephen Henry Hobhouse, and in 1922 they co-authored English prisons to-day: being the report of the Prison system enquiry committee, a devastating critique of the English prison system which resulted in a wave of prison reform which has continued to this day.[17] Brockway wrote over twenty other books on politics and four volumes of autobiography.[10][18][19]

His life and legacy are celebrated in his old constituency of Slough with the now annual FennerFest, a community arts and culture festival. A statue of Fenner Brockway stands at the entrance to Red Lion Square Park in Holborn, London; it was funded by many involved in the Commonwealth independence movements he supported and was expected to be unveiled after his death. However, he achieved such longevity that it was likely that the original Planning Permission to erect it would run out, causing problems to renew the process. It was decided to ask him to unveil it, he being one of the few private individuals, as opposed to Heads of State to do so. It was damaged (an arm was broken off) by a falling tree in the Great Storm of 1987. The refurbished and insured statue was installed shortly after his death. A close in the town of Newport in South Wales is named after him. The 2014 NATO summit will take place just a few hundred yards away.